1. Field of the Invention
The invention relates to a process for producing a layer system with high-quality optical properties and/or a high surface smoothness, and to coated substrates with high-quality optical properties and/or a high surface smoothness.
2. Description of Related Art
Processes for coating substrates with in particular optical layers to produce optical components, such as for example mirrors or reflectors, have long been known. The optical layers have a very wide range of functions in terms of their effect on the radiation within a defined region of the electromagnetic spectrum.
Processes for coating substrates with in particular optical layer systems which are composed of a plurality of individual functional layers, in particular of alternately arranged layers with high and low refractive indices, have likewise been known for years for a wide range of applications. In this context, the layer systems often act as a light interference film, the optical properties of which are determined by the choice of material for the layers with high and low refractive indices and therefore of the corresponding refractive indices, by the arrangement of the individual layers and by the choice of the individual layer thicknesses. The choice is made substantially on the basis of known optical design rules and design tools according to the desired optical properties and also the processing properties.
In recent years, PVD (physical vapor deposition) processes and CVD (chemical vapor deposition) processes have become the main processes used to produce layers and layer systems, in particular for optical layers and layer systems.
CVD processes are used to produce layers of refractory and other metals, carbides, nitrides and oxides. The advantage that it is possible to apply a large number of materials of virtually any theoretical density and a good bonding strength uniformly and with a high level of purity is offset by the disadvantage that suitable reactions do not exist for every desired layer material, the substrate has to be able to withstand the generally high reaction temperature and also has to be chemically stable with respect to the reactant. In general, the pressures which are required during the reaction are from 10 to 100 Pa, and consequently the free path length of the particles is relatively short and the coating rates are not optimal for industrial processes.
Nowadays, by contrast, the PVD processes, in particular sputtering processes, are distinguished by the fact that a wide range of coatable substrate materials are possible, that there is a virtually unlimited choice of coating materials, that the substrate temperature can be selected as desired, that the layer bonding is excellent and that it is easy to influence the microstructure of the layers by selecting the process parameters. The drawbacks of the sputtering processes which were originally developed have been substantially eliminated by extensive development work, and consequently nowadays sputtering technology is among the most universal and widespread coating processes.
In recent years, the use and further development of magnetron sputtering sources has meant that in particular the magnetron sputtering processes have proven suitable for industrial coating processes. The magnetron sputtering processes allow high coating rates in a low pressure range (down to under 0.1 Pa) with little heating of the substrate.
The procedure used in sputtering is fundamentally known to the person skilled in the art.
Substrates are coated by cathode atomization, preferably by magnetron cathode atomization, by means of a sputtering apparatus as described, for example, in DE 41 06 770. Targets, as they are known, of the layer starting material are exposed to the action of a plasma cloud which forms between two electrodes, with the target simultaneously forming the cathode. The atomized target material has an affinity for the reactive gas and, by forming a chemical compound with the latter, is deposited as a layer on the substrate.
EP 0 516 436 B1 has described a magnetron sputtering installation for coating a substrate with one or more layers.
The specific form of the installation contributes to a more efficient sputtering process. For this purpose, the installation has a vacuum chamber in which a substrate holder in drum form and, at the walls of the vacuum chamber, targets of the layer starting materials are located, the targets being arranged on magnetrons.
Rotation of the drum on which the substrates are located causes the substrates to be uniformly coated. This form of sputtering also allows the substrates to be coated with a plurality of layers without having to be taken out of the vacuum chamber, and the thickness of the layer is simple to influence.
However, it has been found that the known sputtering processes for coating substrates with in particular optical layers and layer systems still lead to quality problems in the form of turbidity and relatively extensive roughness of the layer surfaces, which can generally be recognized as regions with diffuse light scattering when the coated substrate is illuminated. In addition to having a purely cosmetic effect, turbidity also reduces the reflectance of the coating and therefore the quality of reflection optics. In the case of filter optics, this turbidity leads to a reduction in the transmittance. In both cases, in addition to the effect of light scattering, increased absorption can also contribute to reducing the product quality.